A catalyst accelerates the rate of a chemical equilibrium reaction by lowering the activation energy of the reaction and is used in a wide range of chemical reaction processes such as synthesis and decomposition processes. Catalysts are categorized into homogeneous catalysts and heterogeneous catalysts. For example, a homogeneous catalyst is composed of a catalytic substance that is dispersed, for example dissolved, in a solvent. The use of the homogeneous catalyst allows for efficient synthesis of a target compound in a liquid phase or the like. A heterogeneous catalyst is composed of a catalytic substance immobilized on a carrier. The heterogeneous catalyst efficiently catalyzes synthesis or decomposition of a target substance and can be easily separated and recovered from the product. Thus, the heterogeneous catalysts are particularly useful in large-scale chemical synthesis plants. An electrode catalyst is a heterogeneous catalyst in which a catalytic substance is immobilized on the surface of an electrode. The electrode catalyst permits an electrochemical reaction to proceed at a lower overvoltage. In particular, the electrode catalysts are needed in fuel cells for the purposes of lowering the overvoltage and generating larger amounts of electrical energy.
Fuel cells are classified into several types according to the electrolytes or the electrodes used therein. Typical types are alkaline types, phosphoric acid types, molten carbonate types, solid electrolyte types and polymer electrolyte types. In particular, polymer electrolyte fuel cells that can operate at temperatures ranging from low temperatures (about −40° C.) to about 120° C. have attracted attention and have been progressively developed and practically used as low-pollution power sources for automobiles. The polymer electrolyte fuel cells are expected to be used as automobile drive sources or stationary power sources. However, the use in these applications requires long-term durability.
The polymer electrolyte fuel cell has a solid polymer electrolyte sandwiched between an anode and a cathode. A fuel is fed to the anode, and oxygen or air is supplied to the cathode, whereby oxygen is reduced at the cathode to produce electricity. The fuel is usually hydrogen or methanol.
To increase the reaction rate in a fuel cell and enhance the energy conversion efficiency, a layer containing a catalyst (hereinafter, also referred to as “fuel cell catalyst layer”) is conventionally provided on the surface of a cathode (an air electrode) or an anode (a fuel electrode) of a fuel cell.
Here, noble metals are generally used as the catalysts. Of the noble metals, platinum that is stable at high potential and has high activity is most frequently used. However, since platinum is expensive and exists in a limited amount, alternative catalysts have been desired.
Further, the noble metals used on the cathode surface are often dissolved in an acidic atmosphere and are not suited in applications requiring long-term durability. Accordingly, it has been strongly demanded that catalysts be developed which are not corroded in an acidic atmosphere and have excellent durability and high oxygen reducing ability.
Materials containing nonmetals such as carbon, nitrogen and boron have captured attention as alternative catalysts to platinum. The materials containing these nonmetals are inexpensive compared to the noble metals such as platinum and are abundant.
Nonpatent Literature 1 reports that zirconium-based ZrOxN compounds show oxygen reducing ability.
Patent Literature 1 discloses, as platinum-alternative materials, oxygen-reducing electrode materials containing a nitride of one or more elements selected from Groups 4, 5 and 14 in the long periodic table.
However, the materials containing these nonmetals do not have sufficient oxygen reducing ability for practical use as catalysts.
Patent Literature 2 considers the possibility for a perovskite oxide containing two or more metals to be used as a platinum-alternative catalyst. However, as demonstrated in Examples, the oxide does not show sufficient activity and only serves as a carrier which assists platinum.
Meanwhile, platinum is useful not only as a fuel cell catalyst as described above but as a catalyst in exhaust gas treatment or organic synthesis. However, the expensiveness and the limited amount of platinum have created a need of alternative catalysts in these applications as well.